The present invention relates to a metamaterial photocathode (MMP) to enable detection of light from visible through long-wave infrared (LWIR) wavelengths (0.4 to 15 μm).
One of the reasons to develop photocathodes outside of the current spectral band is the prolific distribution of low cost visible cameras. These visible sensors use silicon as the active absorber to convert the incoming light to an electrical signal. Silicon has a bandgap of 1.124 eV at 300 K which corresponds to a cut-off wavelength of 1.10 μm which means these sensors can also detect in the near infrared (NIR). Through device engineering and optical coatings one can tune the response of a Si detector/pixel for different bands. The typical focal plane architecture for these imagers is either a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) design. The choice of either a CCD or CMOS along with coating and illumination direction (front side vs back side) can have a significant impact on the spectral response of the sensor. The other trade-offs between these two architectures are read out noise, cost, array size, and dynamic range.
The main problem with moving out of band is lack of infrared sensors that can operate at a high refresh rate and still minimize size, weight, and power consumption (SWaP) of the overall payload. The night vision goggle (NVG) meets these criteria for high refresh rate and low SWaP, but it is limited to operation only in the Vis/NIR. The current state-of-the-art in mid-wave infrared (MWIR) imagers is a solid state sensor using InSb or type-II superlattice semiconductor structures as the absorbing elements. Both of these material technologies require cryogenic cooling between 77 and 150 K. This deep cooling requirement is almost an immediate disqualification for handheld use because of the power and size needed for the cold Dewar and pumping components, but some MWIR systems do exist in a handheld form factor at high cost. Current uncooled LWIR imagers operate with an optimal SNR at a maximum of around 30 frames per second. Any further decrease in the response time of the pixel reduces the SNR below a usable limit. Thus current LWIR imagers are also not viable candidates as an out-of-band solution because they are not capable of operating high paced, high frame rate environment. For the short-wave infrared (SWIR), the dominant imaging technology is a solid state sensor with InGaAs pixels. The primary factor in its determination of use is the fact that it has a relatively high sensitivity and can operate at room temperature. Some tradeoffs for consideration for this technology are the large costs associated with these manufacturing FPAs and the power requirements associated with digital imaging arrays when operated at high frame rates.
According to an illustrative embodiment of the present disclosure, a metamaterial photocathode (MMP) to enable detection of light from visible through LWIR wavelengths. A metal-semiconductor-NEA (MSNEA) is used to enhance electron emission. Plasmonic decay creates a hot electron on the surface of the metal. Excess majority carriers (holes) in the p-type semiconductor diffuse into the metal across the Ohmic contact to balance out the charge. Electron-hole pairs (EHP) are thermally created within the semiconductor as holes leave allowing extraction of electrons into the vacuum off the NEA surface. By incorporating a NEA structure into the MMA, the structure would still be impedance matched for high absorption and also be optimized for electron emission from the surface of the metal. Because the core technology behind the absorption mechanism can be scaled to other absorption bands by size and shape, the only upper limit is shorter wavelengths and the ability to fabricate smaller structures.
Additional features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of the illustrative embodiment exemplifying the best mode of carrying out the invention as presently perceived.